1. Field of the Invention
The present invention relates to a control device for an internal combustion engine, for controlling an air/fuel ratio of an exhaust gas.
2. Related Background Art
In an exhaust path of a general internal combustion engine, a three-way catalyst for simultaneously cleaning HC, CO, and NOx contained in an exhaust gas is provided. The three-way catalyst exhibits a high cleaning rate for any of HC, CO, and NOx when an air/fuel ratio of the exhaust gas is in the vicinity of a stoichiometric air/fuel ratio.
Therefore, generally, an O2 sensor (hereinafter, referred to as an “upstream O2 sensor”) is provided to an upstream side of the catalyst to perform feedback control based on an output from the upstream O2 sensor so that the air/fuel ratio of the exhaust gas becomes closer to the stoichiometric air/fuel ratio.
However, the upstream O2 sensor is provided in the exhaust path as close to a combustion chamber as possible, that is, is attached to a location where the exhaust manifolds are collectively provided. Therefore, the upstream O2 sensor is exposed to the exhaust gas at a high temperature and is poisoned by various toxic substances. Moreover, since the exhaust gas is not sufficiently mixed at the location close to the combustion chamber, a variation occurs in air/fuel ratio of the exhaust gas.
Therefore, there is a problem in that the output from the upstream O2 sensor greatly fluctuates, making it impossible to accurately control the air/fuel ratio of the exhaust gas.
In order to solve the above problem, a double-O2 sensor system including an O2 sensor (hereinafter, referred to as a “downstream O2 sensor”) provided to a downstream side of the catalyst in addition to the upstream O2 sensor has been proposed.
In the double-O2 sensor system, the feedback control is performed on the air/fuel ratio based on the output from the upstream O2 sensor as described above. At the same time, the feedback control is also performed on the air/fuel ratio of the exhaust gas based on the output from the downstream O2 sensor.
Although a response speed of the downstream O2 sensor is lower than that of the upstream O2 sensor, the passage of the exhaust gas through the catalyst lowers an exhaust temperature to reduce the effects of heat. In addition, toxic substances are absorbed by the catalyst to reduce the effects of the toxic substances. Moreover, since the exhaust gas is sufficiently mixed on the downstream side of the catalyst, the air/fuel ratio of the exhaust gas is equilibrated.
Therefore, the double-O2 sensor system makes it possible to absorb an output fluctuation of the upstream O2 sensor and keep high cleaning rate of catalyst by controlling the output of the downstream O2 sensor to the target.
Moreover, oxygen storage capacity is imparted to the catalyst to absorb a temporary fluctuation in air/fuel ratio of the exhaust gas on the upstream side of the catalyst. The oxygen storage capacity plays a role of an integrator for taking in and storing oxygen in the exhaust gas when the air/fuel ratio of the exhaust gas is on the lean side of the stoichiometric air/fuel ratio and for releasing the stored oxygen when the air/fuel ratio of the exhaust gas is on the rich side of the stoichiometric air/fuel ratio.
Therefore, the fluctuations in air/fuel ratio on the upstream side of the catalyst are averaged in the catalyst, whereby an average air/fuel ratio acts on a catalyst cleaning state. Thus, in order to maintain a satisfactory cleaning rate of the catalyst, the output from the downstream O2 sensor is used to control the average value of the air/fuel ratio of the exhaust gas on the upstream side of the catalyst.
A conventional air/fuel ratio control device for an internal combustion engine changes a controlling constant of feedback control using the output from the upstream O2 sensor in accordance with the output from the downstream O2 sensor to control the average air/fuel ratio on the upstream side (for example, see JP 63-195351 A).
In the above-described conventional device, as the controlling constant used for the feedback control (first air/fuel ratio feedback control means) using the output from the upstream O2 sensor, at least one of a delay time, a skip amount, an integral constant, and a relative voltage is included. It is possible to control the average air/fuel ratio by setting each of the delay time, the skip amount, and the integral constant asymmetrically when air/fuel ratio is controlled on the rich side or the lean side, and also by changing the relative voltage.
Specifically, for example, by setting: the delay time on the rich side>the delay time on the lean side, the average air/fuel ratio shifts to the rich side. On the contrary, by setting: the delay time on the lean side>the delay time on the rich side, the average air/fuel ratio shifts to the lean side.
By setting: the skip amount on the rich side>the skip amount on the lean side, the average air/fuel ratio shifts to the rich side. On the contrary, by setting: the skip amount on the lean side>the skip amount on the rich side, the average air/fuel ratio shifts to the lean side.
In the same manner, by setting: the integral constant on the rich side>the integral constant on the lean side, the average air/fuel ratio shifts to the rich side. On the contrary, by setting: the integral constant on the lean side>the integral constant on the rich side, the average air/fuel ratio shifts to the lean side.
By increasing the relative voltage, the average air/fuel ratio shifts to the rich side. By decreasing the relative voltage, the average air/fuel ratio shifts to the lean side.
As described above, the above-described controlling constants are calculated based on the output from the downstream O2 sensor to control the average air/fuel ratio of the exhaust gas on the upstream side of the catalyst for one control cycle.
Moreover, the simultaneous control of two or more of the above-described controlling constants to improve the controllability of the average air/fuel ratio has also been proposed.
In the above-described conventional device, however, a common management index is not set. Therefore, if merely two or more of the controlling constants are simultaneously controlled, a non-linear interaction occurs.
Therefore, when the air/fuel ratio of the exhaust gas on the upstream side of the catalyst is to be shifted to the lean side or the rich side, the control of the amount of shifting the average air/fuel ratio (a shift amount) becomes difficult even though the direction of shifting the average air/fuel ratio (a shift direction) can be controlled.
The above-mentioned non-linear interaction occurs by the mutual influences of changes of the controlling constants. Therefore, the shift amount of the average air/fuel ratio when two or more controlling constants are simultaneously controlled does not become equal to the result of a simple addition of the shift amounts when each of the controlling constants is controlled alone. The shift amount of the average air/fuel ratio is varied depending on the amount of control when each of the controlling constants is controlled, the combination and the points of operation of the controlling constants, the characteristics of a control target, which vary depending on operating conditions, or the like.
The non-linear interaction is also caused by the non-linear relation between the amount of control of each of the controlling constants and the shift amount of the average air/fuel ratio.
In the conventional control device for an internal combustion engine, the shift amount of the average air/fuel ratio varies depending on the amount of control of each of the controlling constants, the combination and the points of operation of the controlling constants, the operating conditions, and the like, which varies a gain of the feedback control.
Therefore, there arises a problem in that hunting or insufficient following occurs to destabilize the feedback control for controlling the average air/fuel ratio of the exhaust gas on the upstream side of the catalyst in accordance with the output of the downstream O2 sensor.
Each controlling constants have each advantages and disadvantages for control of the average air/fuel ratio, such as, a control accuracy of the average air/fuel ratio, a control range of the average air/fuel ratio, a control cycle, control amplitude of the air/fuel ratio oscillation and the like.
It is conceivable to effectively combine the controlling constants to utilize each advantage and moderate each disadvantages.
In the conventional control device for an internal combustion engine, however, a common management index is not set.
Therefore, there is another problem in that the amount of control of the controlling constants or the combination of the constants cannot be determined in detail to maximize each advantage and suppress each disadvantage in accordance with the point of operation of the average air/fuel ratio.